1. Field of the Invention
The present invention relates to a substrate for use to make a thin-film magnetic head slider for a hard disk drive (which will be referred to herein as a “thin-film magnetic head substrate”) and also relates to a method of manufacturing such a substrate.
2. Description of the Related Art
Thanks to recent tremendous development of information and telecommunication technologies, the amount of information that can be processed by computers has increased by leaps and bounds. In particular, audiovisual (or multimedia) information such as sounds, music and video, which used to be capable of being processed only as analog signals, now can be converted into digital signals and processed by personal computers. Multimedia data such as music and video contains a huge amount of information. Thus, it has become more and more necessary to further increase the capacity of storage media for use in personal computers, for example.
A hard disk drive is a typical information storage device that has been used broadly in personal computers, for example. To meet the demand described above, the capacity of hard disks needs to be further increased and the overall size of the drive needs to be reduced.
FIG. 7 is a cross-sectional view schematically illustrating a thin-film magnetic head slider and surrounding portions thereof in a conventional hard disk drive. As shown in FIG. 7, an undercoat film 13 is provided on a side surface of a base 12, which is supported on a gimbal 10. A read/write head 14′ is provided on the undercoat film 13. Such a unit, including the base 12 and read/write head 14′ to be supported on the gimbal 10, is normally called a “head slider” or simply “slider”.
The read/write head 14′ is made of a magnetic material and has a notched ring configuration, inside which a coil 15 is wound. When a recording signal is supplied to the coil 15, a magnetic field is generated in the read/write head 14′, thereby writing data on a magnetic storage medium 17. Also, the read/write head 14′ senses a variation in the magnetic field on the magnetic storage medium 17 and converts the sensed variation into an electric signal representing the data that is stored on the magnetic storage medium 17.
FIG. 8 illustrates another conventional thin-film magnetic head slider. The thin-film magnetic head slider shown in FIG. 8 includes two separate magnetic heads, i.e., a read head 16 and a write head 14, to perform the read operation more efficiently. The structure of the write head 14 is similar to that of the read/write head 14′ shown in FIG. 7 and is used exclusively to write data on the magnetic storage medium 17.
On the other hand, the read head 16 is a magneto-resistive (MR) or giant MR (GMR) element to convert a variation in magnetic field into a variation in electrical resistance. That is to say, the read head 16 senses a variation in the magnetic field recorded on the magnetic storage medium 17, thereby producing an electric signal representing the data that is stored on the magnetic storage medium 17.
The base 12 to hold the read/write head 14′ or the read head 16 and the write head 14 thereon has often been made of an Al2O3—TiC based ceramic sintered body. The Al2O3—TiC based ceramic material (which will be herein referred to as an “AlTiC material”) has been used extensively because this material exhibits excellent thermal and mechanical properties and processibility while striking an adequate balance between them. However, the AlTiC material is a good electrical conductor. Accordingly, if the read/write head 14′ or write head 14 was disposed adjacent to such a conductor base 12, then the read/write head 14′ or the write head 14 would be short-circuited and could not operate properly. Also, the surface of such an AlTiC base has pores and is not sufficiently smooth. For that reason, to electrically insulate the read/write head 14′ or write head 14 from the base 12 sufficiently and increase the smoothness of the surface of the base 12, the undercoat film 13 of Al2O3 is normally provided on the side surface of the base 12. This is because Al2O3 exhibits a good insulation property and has a smooth enough surface.
Recently, however, it has become more and more necessary to further increase the storage capacity of hard disks and yet reduce the overall size of hard disk drives as described above. To meet these demands, a slider having a different structure was proposed. In the proposed structure, the read and write heads 16 and 14 are arranged in reverse order as shown in FIG. 9A. That is to say, the read head 16 is disposed adjacent to the undercoat film 13 and the write head 14 is spaced apart from the base 12.
To realize a hard disk drive including a slider with the newly proposed structure, however, various problems must be overcome.
Firstly, as it has become more and more necessary to reduce the overall size of hard disk drives, sliders also must be further reduced in size. To reduce the size of sliders, the cross-sectional area of the coil 15 inside the write head 14 should be reduced as shown in FIG. 9B. More specifically, the inside diameter of the coil 15 needs to be minimized and yet respective loops of the coil 15 should not overlap with each other. However, when a current flows through the coil 15 with such a reduced cross-sectional area by way of terminals 18, the quantity of heat generated per unit area increases.
However, Al2O3, which has often been used as a material for the undercoat film 13, does not have such good thermal conductivity. Accordingly, the heat, generated by supplying the coil 15 with current, is shut off by the Al2O3 undercoat film 13, and cannot diffuse toward the base 12 sufficiently. Thus, the heat is stored in the read head 16 or the write head 14. As a result, the read head 16 or the write head 14 thermally expands and protrudes toward the magnetic storage medium 17 as indicated by the arrow in FIG. 9A. The gap between the read head 16 or the write head 14 and the magnetic storage medium 17 is as small as about 10 nm. Thus, it is quite possible for the thermally expanded read head 16 or write head 14 to contact with the magnetic storage medium 17 accidentally.
This phenomenon is called a “thermal pole tip recession (TPTR)”. When this phenomenon happens, the read head 16 or the write head 14 physically contacts with the magnetic storage medium, thereby scratching the magnetic storage medium or damaging the read head 16 or the write head 14 itself. Then, a serious failure might be caused, or the hard disk drive could not operate properly anymore.
Also, even if the read head 16 or the write head 14 can avoid contact with the magnetic storage medium 17, the gap between the read head 16 or the write head 14 and the magnetic storage medium 17 changes due to the thermal expansion of the read head 16 or the write head 14. For example, when the read head 16 or the write head 14 expands several nanometers, the gap between the magnetic storage medium 17 and the read head 16 or the write head 14 decreases 10% or more. Then, the read or write performance is affected significantly, and errors may be created in the signals to be written on, or read out from, the magnetic storage medium.
To overcome this problem, the undercoat film 13 may have a reduced thickness so that the heat can be radiated into the base 12 more easily. In that case, however, the pores on the surface of the base 12 would decrease the surface smoothness of undercoat film 13, thus making the undercoat film 13 almost meaningless.
Furthermore, because the size of the read head 16 or the write head 14 has been decreased for the reasons described above, it becomes more and more probable that electrostatic breakdown is caused in the read head 16 and the write head 14 by static electricity generated. Static electricity is generated particularly easily in a hard disk drive because the rigid magnetic storage medium rotates at a high velocity. On the other hand, the read head 16 and the write head 14 are provided on the undercoat film 13 of Al2O3 with high electrical insulating property. As a result, the static electricity generated is likely stored in the read head 16 and the write head 14. When the quantity of the static electricity stored exceeds a predetermined limit, the static electricity stored is discharged at a time, thus causing electrostatic breakdown on the read head 16 and the write head 14.
This problem also seems to be solved if the undercoat film 13 has a reduced thickness and lower insulating property. In that case, however, the static electricity stored could be discharged into the AlTiC base 12 that is adjacent to the undercoat film 13. Then, dielectric breakdown could occur in the undercoat film 13.